The present invention relates to an electronic ballast, especially a self-oscillating resonant inverter. The ballast is used in a power supply, for example, in a DC to AC inverter for energizing an inductively coupled gas discharge lamp.
A gas discharge lamp typically utilizes an electronic ballast for converting AC line voltage to high frequency current powering a gas discharge lamp. The ballast usually includes a resonant inverter converting DC bus voltage to lamp high frequency current. The resonant inverter comprises at least one switching transistor generating high frequency rectangular AC voltage and a resonant load having an inductor and a capacitor in series. The gas discharge lamp is coupled in parallel to the capacitor. For high frequency lamps (up to 2.65 MHz and higher), it is common that a self-oscillating inverter generates high voltage at the resonance frequency for lamp instant starting. It is known that a self-oscillating inverter with a voltage feedback is preferred for electrodeless lamps as is the case with products identified in connection with the marks ICETRON, QL, GENURA, and EVERLIGHT. This self-oscillating inverter utilizes a feedback transformer coupled to the inverter output via a feedback capacitor used for driving inverter switching MOSFETs. By means of the voltage feedback circuit, a sinusoidal voltage across the gates of switching MOSFETs is generated. Therefore, dead time intervals for the switching transistors are automatically formed when crossing near zero gate voltage. This voltage feedback for ballast inverters is described, for instance, in U.S. Pat. No. 4,748,383 issued to Houkes, U.S. Pat. No. 5,962,987 issued to Statnic and U.S. Pat. No. 5,982,108 issued to Buij at al.
It is known in the field that a few factors have important influence on starting transients. First, during lamp starting, when the Q of the resonant load is high, the output ballast voltage is much higher than when the Q of the resonant load is in the steady-state mode. Actual resonant frequency of the parallel loaded resonant circuit is slightly higher when Q is high (before the lamp is lit), than after lamp starting. This factor is neglected in calculations of the resonant frequency, but must be taken into account for a system that runs close to resonance.
Second, lamp and ballast inductors may saturate in one direction during starting by low frequency current components and may increase resonant frequency at these saturation intervals. At constant switching frequency, the resonant load can turn capacitive and the resonant load then operates in a capacitive mode.
Third, during starting, the amplitude of the transistor gate voltage in the self-oscillating inverter is increased. U.S. Pat. No. 5,349,270 issued to Roll et al. teaches clamping gate voltage using a pair of back-to-back connected Zener diodes to limit gate voltage during lamp starting. But, clamping these Zener diodes during lamp starting creates a transient phase delay of the feedback signal that contributes to the capacitive mode of operation mentioned above.
The fourth drawback is a gate voltage slope change during starting of self-generating ballast inverter. During lamp starting the dead time intervals of the switching transistors are reduced, causing an increase in the rms voltage applied to the resonant load and an even higher output ballast voltage.
When the resonant load temporarily changes from an inductive to a capacitive nature by all the above factors acting together, a switching transistor can be turned ON when the body diode of another transistor is conducting resonant load current. This creates a high reverse recovery current in the body diode that will destroy the switching transistor with time (see T. Wu and C. Nguyen, xe2x80x9cDynamic Stresses Can Cause Power MOSFET Failuresxe2x80x9d, PCIM, April 2000, p.28).
In general, the inverter feedback circuit could be tuned up with pre-advanced switching angle, so that the inverter will start the lamp without cross conduction in the MOSFETs. But, this arrangement does not provide the most efficient optimized steady-state mode in the ballast inverter.
FIG. 1a shows a typical, prior art electronic ballast arrangement for, preferably, an electrodeless lamp. This arrangement is effective for converting a standard AC line voltage to high frequency current for driving the lamp. The ballast AC to DC converter derives AC from the power line through an EMI filter, rectifies the AC, and optionally corrects the power factor. The AC to DC converter output voltage is filtered out by an electrolytic capacitor C25 connected across a high voltage DC bus.
The self-generating ballast inverter is connected to the DC bus and its output is connected to the lamp. A high frequency capacitor C31 reduces high frequency voltage ripple on the DC bus. Two switching MOSFETs, designated as M1 and M2, are coupled in series across the DC bus. A resonant load comprises, in series, an inductor L1, a capacitor C3, and the lamp coupled in parallel to the capacitor C3. The resonant load is connected in parallel to the switching MOSFET M2 via a DC blocking capacitor C1. The switching transistors, M1 and M2, are driven by a feedback circuit.
The feedback circuit includes a capacitor C27, a feedback transformer T9, a compensating capacitor C30, coupled in parallel to the transformer T9, and MOSFETs M1 and M2. The gates of M1 and M2 are coupled to secondary windings of the transformer T9 via resistors R16 and R15, respectively. Zener diodes D46 and D51 are connected back-to-back in series and clamp the gate voltage of the transistor M1. Zener diodes D52 and D53 provide clamping for the gate voltage of the transistor M2. Clamping gate voltages helps to protect the feedback transformer T9 from saturation during the lamp starting.
Also, the inverter in FIG. 1a comprises a start circuit with a storage capacitor C29, charged from the DC bus via a resistor R19, a diac X28 for generating a start signal, and a dummy resistor R23 for charging DC blocking capacitor C1 before starting the inverter. A diode D10 discharges the storage capacitor C29 to prevent the inverter from restarting during steady-state operation. Rectangular AC voltage V1 with near to resonant frequency is applied to the resonant load, resulting in sinusoidal voltage Vout across the lamp (see FIG. 1a).
FIG. 1b shows voltage versus frequency plots for the resonant load at a lamp-starting mode 1 and at lamp-lit mode 2 (steady-state). In the conventional prior art circuit of FIG. 1a, the switching frequency fsw is about the same during both modes 1 and 2. But, the resonant frequency fr1 in mode 1 is higher than the resonant frequency fr2 in mode 2 (fr1 greater than fr2 in FIG. 1b). Therefore, if the inverter switches above resonant frequency (fsw greater than fr2) in optimized zero-voltage-switching (ZVS) mode 2, the inverter may switch below resonant frequency (fsw less than fr1) in mode 1 with high reverse recovery currents in the body diode of MOSFETs M1 and M2. Operating points of the inverter in both modes are shown by dots in FIG. 1b. 
One way to avoid problems between transient mode and steady-state mode requirements is to use a Phillips 85 W QL electrodeless lamp ballast having a self-generating inverter with an additional feedback circuit. Using a regulated transistor, this feedback circuit has an additional frequency dependent network connected across the feedback transformer. This type of circuit arrangement is described in U.S. Pat. No. 5,550,438 issued to Reijnaerts. However, the voltage limit feedback circuit in the Reijnaerts patent senses only a positive wave of lamp voltage. Thus, the Reijnaerts feedback circuit is not stable enough and also requires too many components, thereby shortening the life of the ballast. Therefore, a need still exists for improving ballast inverters powering electrodeless gas discharge lamps.
It is an object of the present invention to provide a ballast inverter with a feedback circuit that gradually advances the phase angle of the feedback signal versus lamp voltage, and, correspondingly, corrects switching frequency to avoid cross conduction in inverter switching transistors during lamp starting.
It is another object of the present invention to provide a ballast inverter circuit that incorporates a voltage feedback circuit that can be economically built with readily available passive electronic components.
It is yet another object of the invention to obtain an inverter circuit with limited transistor gate output starting voltage and reduce voltage stress on resonant load components.
It is still another object of the invention to generate a signal that shuts off the inverter if the lamp fails to start or the lamp is damaged.
It is still further an object of the invention to provide a ballast inverter having a life comparable with the relatively long life of an electrodeless gas discharge lamp.
In accordance with the invention, claimed herein, there is provided a ballast inverter circuit comprising a resonant load. When the ballast inverter circuit is used with a gas discharge lamp, the resonant load includes the lamp. The ballast inverter circuit comprises a switch, preferably at least one transistor switch, but more preferably two transistor switches serially connected across the output of a DC power supply, for instance, an AC to DC converter. The resonant load comprises an inductor and resonant capacitor connected in series between a common node of the transistor switches and an output terminal of the DC power supply, via a DC capacitor. The lamp is coupled in parallel to the resonant capacitor.
The ballast inverter circuit also comprises a voltage feedback circuit and a start circuit. The voltage feedback circuit includes a feedback transformer having a primary winding coupled to the inverter output via two series feedback capacitors. Each secondary winding controls a MOSFET switch through its gate network. A bi-directional voltage clamp is connected in series with a phase shift resistor and this series circuit is connected in parallel to the primary winding and the feedback capacitor connected in series.